I hope someone can help me, I am unable to start up any instance.
I am using centos 6.4 x86_64 and Openstack Grizzly, KVM+glusterfs with 2 compute nodes and 1 controller.
This problem happened both with qcow2 images and raw images. I can confirm that when running guestmount manually, there is no problem.
Selinux is disabled.

Failed to mount filesystem: Unexpected error while running command.
Command: sudo nova-rootwrap /etc/nova/rootwrap.conf guestmount --rw -a /var/lib/nova/instances/instance-00000008/disk -i /tmp/openstack-disk-mount-
tmpoeXM1j
Exit code: 1
Stdout: ''
Stderr: "/usr/bin/guestmount: no operating system was found on this disk\n\nIf using guestfish '-i' option, remove this option and instead\nuse the commands 'run' followed by 'list-filesystems'.\nYou can then mount filesystems you want by hand using the\n'mount' or 'mount-ro' command.\n\nIf using guestmount '-i', remove this option and choose the\nfilesystem(s) you want to see by manually adding '-m' option(s).\nUse 'virt-filesystems' to see what filesystems are available.\n\nIf using other virt tools, this disk image won't work\nwith these tools. Use the guestfish equivalent commands\n(see the virt tool manual page).\n\nRHEL 6 notice\n-------------\nlibguestfs will return this error for Microsoft Windows guests if the\nseparate 'libguestfs-winsupport' package is not installed. If the\nguest is running Microsoft Windows, please try again after installing\n'libguestfs-winsupport'.\n")

Comments

A few more attempts, with a raw image, I am getting:
"Command: sudo nova-rootwrap /etc/nova/rootwrap.conf losetup --detach /dev/loop0
Exit code: 1
Stdout: ''
Stderr: "loop: can't delete device /dev/loop0: Device or resource busy\n")"
An ip has been assigned, but in console I can see the "Boot failed: could not read the boot disk" message